In this example, a monosynaptic reflex is elicited by electrical stimulation of the group Ia fibers in each of two nerves (see
Fig. 9.11A ). The reflex response is characterized by a recording of the discharges of α motor axons from the appropriate ventral root (as a compound action potential). When nerve A is stimulated, a small compound action potential is recorded as reflex A. Similarly, when nerve B is stimulated, reflex B is recorded.
Fig. 9.11B depicts the motor neurons contained within the motor nucleus.
CHAPTER 9 Organization of Motor Function •Fig. 9.10 Phase Reset of Locomotion Rhythm by Flexion Reflex Afferent (FRA) Fiber Stimulation Helps Identify Neuronal Components of the Underlying Central Pattern Generator (CPG). A, EMGrecordsfromkneeflexorandextensormuscles.Notetherhythmicalternatingpatternbeforeapplicationofthestimulus.Thesolid vertical lines beloweachtraceindicatethetimesatwhichflexorcontractionisinitiated.Thedashed vertical lines indicatethetimesatwhichflexorcontractionwouldhavebeeninitiatedifthestimuluscausednolastingeffectontherhythmicpattern.B andC, TwopossiblemodelsfortheCPGunderlyingthelocomotorrhythmdepictedinA. B, TheFRAinterneurons(INs)intheCPGarenotshown.C, TheFRAinterneuronsareshown.ThedatashowninA supportthemodelshowninC. MN,motoneuron.(DatafromHultbornH,etal.Ann N Y Acad Sci. 1998;860:70.)
The α motor neurons in the discharge zones are activated above threshold when each nerve branch is stimulated separately. Thus a distinct pair of α motor neurons spike when each nerve is stimulated alone. In addition, each of these motor neuron pairs is surrounded by a subliminal fringe of eight additional motor neurons that are excited but not sufficiently to trigger spikes. When the two nerves are stimulated at the same time, a much larger reflex discharge is recorded (compare RA and RB with RA + RB recordings at the right of
Fig. 9.11B ). As the figure demonstrates, this reflex represents the discharge of seven α motor neurons: the four that spiked after the singular stimulation of each nerve (two per nerve) and three additional α motor neurons (located in the facilitation zone) that are made to discharge only when the two nerves are stimulated simultaneously because they lie in the subliminal fringe for both nerves.
A similar effect could be elicited by repetitive stimulation of one of the nerves, provided that the stimuli occur close enough together that some of the excitatory effect of the first volley still persists after the second volley arrives. This effect is called temporal summation. Both spatial summation and temporal summation depend on the properties of the excitatory postsynaptic potentials evoked in α motor neurons by the group Ia afferent fibers (see
Fig. 6.8).
Convergence can also lead to inhibitory interactions between stimuli, a phenomenon called occlusion. If a volley in one of the two nerves in
Fig. 9.11 reaches the motor nucleus at a time when the motor neurons are highly excitable, the reflex discharge is relatively large (see
Fig. 9.11C ). A similar volley in the other nerve might also produce a large reflex response. However, when the two nerves are excited simultaneously, the reflex can be less than the sum of the two independently evoked reflexes if the cells reaching threshold to activation of either of the two nerves alone overlap significantly. In this case, each afferent nerve activates 7 α motor neurons, but the volleys in the two nerves together cause only 12 α motor neurons to discharge because two motor neurons lie in the individual discharge zones of both afferent nerves.